Stress instrumentation, sensitivities

Unfortunately the high Peclet number regime is where many rheological measurements are most easily made. High stresses and strain rates allow the development of simpler instrumental designs and lower sensitivities are required. It is also important to be aware of the fact that many applications require very high deformation regimes and it is... 

Dynamic mechanical testers apply a small sinusoidal stress or strain to a small sample of the polymer to be examined and measure resonant frequency and damping versus temperature and forced frequency. Instrument software computes dynamic storage modulus (G ), dynamic loss modulus (G") and tan delta or damping factor. Measurements over a wide range of frequency and temperature provide a fingerprint of the polymer with sensitivity highly superior to DSC. 

As yet, few attempts have been made to study the effects of pharmacological challenge in individual primates that have been socially stressed or that are temperamentally vulnerable. The success of this approach is illustrated by an important study (Insel et al. 1988) in which peer-reared rhesus monkeys were shown to be especially sensitive to a p-CCE challenge and by our own studies using a behavioral test battery as a screening instrument to identify vulnerable and resistant subjects before administration of CCK-4 (Palmour et al. 1992a). 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

The dynamic mechanical method, however, suffers from two important limitations. Firstly, it is not sensitive for blends having less than 10 wt.- % of one component, as the response is dependent on the fraction of the stress born by each phase. Secondly, instruments have poor resolution for blends of materials with close T s. Different machines obviously have different sensitivities in both these respects. 

Details on the instruments available for collecting PL spectra are reported in refs. [11, 78]. One of the main points to be stressed is that, because of the high sensitivity of the PL technique, the sample cell has to be made from high-quality fused silica with no impurities, such as Suprasil. 

The application of ultraviolet and visible spectroscopy to the identification and measurement of carbenium ions derived from aromatic and dienic monomer has already been discussed (see Sect. II-G-2). The use of this technique to monitor stable carbenium salts is also well known. We have finally stressed in a preceding section that the fate of certain anions could be followed spectrophotometrically during a cationic polymerisation. The limits of detection allowed by the values of the extinction coefficients of all these species and by the sensitivity of present-day instruments is 10 to 10 M. 

The creep function J(t) is the transient strain per unit stress in a step-stress experiment. The resolution at short times is also limited from instrument response and sensitivity. J(t) at short times may also be derived from the high frequency complex compliance data. 

The melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

A more recent, and very promising, development is due to Cohen and James, who adapted a position-sensitive proportional counter to stress measurement. This counter has the great advantage that it can measure the 29 position of a diffracted beam without a 29 movement of the counter (Sec. 7-5). Preliminary work was done with the counter mounted on a standard diffractometer [16.10]. Later a portable instrument was made, in which the counter and a miniature, air-... 

---